The (meth)acrylic acid absorption methods described use, as an absorption solvent, either water, or an organic solvent, which is usually a hydrophobic compound or a mixture of hydrophobic compounds with a much higher boiling point than that of (meth)acrylic acid.
Absorption methods using water provide an aqueous (meth)acrylic acid solution which requires numerous and costly purification steps to obtain pure (meth)acrylic acid.
On the contrary, absorption methods using hydrophobic organic solvents have the advantage over aqueous methods of reducing the number of purification steps necessary for obtaining pure (meth)acrylic acid. These methods conventionally involve the successive steps of absorption, stripping, removal of light compounds, followed by final distillation of the pure acrylic acid.
The present invention relates to the substantially quantitative recovery (recovery yield>98.5%.), in only three steps of (meth)acrylic acid that is sufficiently stripped of its light impurities to avoid an additional topping step. A second objective of the invention is to achieve this recovery without dilution of the uncondensed waste gas by an external gas added in the stripping step, in order to reduce the size of the column and the loss of unconsumed reactants in the waste gas that must be purged. The third objective is the recovery of pure (meth)acrylic acid without any aqueous pollutant liquid release that is difficult to remove.
The main method for synthesizing acrylic acid uses a reaction of catalytic oxidation of propylene with a mixture containing oxygen. This reaction is generally carried out in the vapor phase, usually in two steps, which may be carried out in two distinct reactors or a single reactor:                the first step carries out the substantially quantitative oxidation of the propylene to an acrolein rich mixture, in which the acrylic acid is a minority component;        the second step completes the conversion of acrolein to acrylic acid.        
The gas mixture issuing from the second oxidation step consists of:                acrylic acid;        light compounds incondensable in the temperature and pressure conditions commonly employed (unconverted nitrogen, oxygen and propylene, propane present in the reactive propylene, carbon monoxide and dioxide formed in small quantities by final oxidation);        light condensable compounds, particularly water, generated by the propylene oxidation reaction, unconverted acrolein, light aldehydes, such as formaldehyde and acetaldehyde, and acetic acid, the main impurity generated in the reaction section;        heavy compounds: furfuraldehyde, benzaldehyde, maleic anhydride, etc.        
The method for synthesizing (meth)acrylic acid by oxidation is identical in principle to that of acrylic acid, except for the reactive substrate (which may be isobutene or tert-butanol), the intermediate oxidation product (methacrolein) and the types of light condensable byproduct compounds (the reaction gas mixture contains acrylic acid in addition to the light compounds present in the reaction gas of the acrylic acid synthesis method).
The second stage of manufacture consists in recovering the acrylic acid from the hot gas mixture, previously cooled to a temperature of 150-200° C., by introducing this gas at the bottom of an absorption column where it meets a countercurrent flow of solvent introduced at the top of the column, and inside which cooling, condensation, absorption and rectification processes take place simultaneously.
In most of the methods described, the solvent employed in this column is water or a high boiling point hydrophobic solvent.
Regardless of the solvent used, the known methods generally involve:                an absorption column, supplied at the top with solvent, at the bottom of which the reaction gas mixture is introduced, comprising a lower cooling section and in which the gas upflow undergoes partial condensation by meeting a descending liquid mixture stream generally cooled through a heat exchanger, and an upper section designed to absorb the maximum of acrylic acid in the solvent;        a desorption column, supplied with the bottom stream from the absorption column. The role of this column is selectively to remove most of the light compounds absorbed in the preceding step, particularly the acrolein unconverted in the reaction step.        
The lighter uncondensed compounds issuing from the reaction gas are removed at the top of the absorption column.
In order to recover part of the unconverted reactants present in this stream, such as propylene and acrolein, in the case of the method for synthesizing acrylic acid, or isobutene and methacrolein in the case of a method for manufacturing methacrylic acid, part of the uncondensed gas stream at the top of the absorption column is generally recycled to the reaction step, the remainder being purged to prevent the holdup of byproducts in the gas loop thus formed.
The maximum proportion of uncondensed gas recycled to the reaction is also limited by economic criteria: since the quantity of gas fed to the reactors is limited by catalyst performance, the (meth)acrylic acid productivity is decreased by dilution of the reactive substrate.
Since (meth)acrylic acid is sensitive to polymerization promoted by high temperatures, the operating temperature in the desorption column is generally limited, either by carrying out this distillation at the temperature of the mixture under reduced pressure, or by introducing an inert gas at the bottom of a stripping column operating under atmospheric pressure or reduced pressure, the two methods serving to decrease the vapor pressure of the condensable compounds, and consequently the temperature of the liquid-gas equilibria governing the separation.
In the case of absorption methods using water as an absorbent solvent, the raw (meth)acrylic acid mixture recovered at the bottom of the desorption column contains a high proportion of water, about 30-50% by weight. Certain polar compounds, which display a strong affinity for this solvent, such as carboxylic acids, are absorbed in the water. This is particularly the case of acetic acid (case of the synthesis of acrylic acid), or of acetic acid and acrylic acid (the case of methacrylic acid synthesis) formed by a side reaction during the reaction step, which passes completely into this raw (meth)acrylic acid stream.
The separation of the majority impurities, that is, water and acetic acid (case of the synthesis of acrylic acid) or water, acetic acid and acrylic acid (case of the synthesis of methacrylic acid), in order to obtain pure acrylic acid or pure methacrylic acid respectively, is difficult. It requires a large number of separation columns. The dehydration step is generally carried out in the presence of a solvent immiscible with water, in an extraction column or heteroazeotropic distillation column, generally coupled with a column for recovery of the solvent partially solubilized in the extracted aqueous phase, in order to recycle it upstream of the method. The step of removal of the light compounds (topping) usually employs one or two columns, and the step of separation of the heavy compounds (tailing) is carried out in a final column, from which the pure (meth)acrylic acid is extracted at the top.
Absorption methods using a nonaqueous solvent have the advantage of reducing the purification steps, particularly by avoiding the use of heavy and costly water separation methods, the use of hydrophobic solvents making it possible to remove the water at the top of the absorption column.
A further advantage of the nonaqueous absorption methods described, over the water absorption method, is to facilitate the removal of the light compounds.
The role of the desorption column located downstream of the absorption column is to reduce the contents of light condensable compounds in the bottom stream of the absorption column, particularly unconverted acrolein, residual water and acetic acid (case of the synthesis of acrylic acid), or methacrolein, water, acrylic acid and acetic acid (case of methacrylic acid).
However, this removal of the light impurities is not complete in the methods described in the prior art. Thus, the method for purifying acrylic acid with absorption by a mixture of diphenyl and diphenyl ether, described in French patent FR-B-2 146 386, yields a raw acrylic acid still containing 0.5% by weight of acetic acid and 0.5% by weight of water. U.S. Pat. No. 5,426,221, describing a method with absorption of acrylic acid by a mixture of diphenyl, diphenyl ether and dimethyl phthalate, serves to improve the removal of water (representing 0.04% by weight of the raw acrylic acid distilled in the example), but it still leaves 0.26% by weight of acetic acid in the purified acrylic acid without an additional topping step. The method of absorption by carboxylic esters described in French patent FR-B-2 196 986 serves to obtain a grade of acrylic acid still containing 0.3% by weight of acetic acid 0.2% by weight of water.
The grades of acrylic acid obtained by this method are insufficient, in the absence of supplementary purification, for the use of the monomer in its conventional applications. To improve the removal of the light compounds without adding an additional column, solutions have been proposed, consisting in carrying out the topping in an upper section added to the final acrylic acid distillation column. Thus, European patent EP-B-706 986 mentions a recovery column in which the acrylic acid is obtained by a side drawoff, the upper section of the column being used to concentrate the residual light compounds at the top, in order to remove them. The major drawback of such a method is the difficulty of separating the light compounds, particularly acetic acid, thereby requiring a significant increase in the number of trays of the column and condensed flow rate returned to the top (reflux) to ensure the separation and reduce the loss of acrylic acid. This causes a substantial increase in the size of the column, and hence in the investment cost, where, at equivalent column size, a decrease in the column distillation capacity. Furthermore, this system generates a stream still containing AA which, to avoid its loss, must be recycled to a preceding step. This makes the method even more complex and limits the capacity of the column receiving this stream.
A further drawback of absorption methods using heavy hydrophobic solvents is the fact that large quantities of solvent are needed to absorb all the acrylic acid present in the reaction gases. In French patent FR-B-2 146 386, to reach a sufficient recovery rate and avoid costly losses of unabsorbed acrylic acid, the mass ratio of solvent (mixture of diphenyl and diphenyl ether) to acrylic acid is about 9/1, or a concentration of acrylic acid in the raw mixture after absorption and desorption, of about 10%. The consequences of the large size of the columns and ancillary equipment and storage units, higher energy costs associated with vaporization of high boiling point solvent in the columns.
Moreover, the absorption methods using nonaqueous solvents described in the literature have the common feature of carrying out the desorption step by stripping by an inert gas introduced at the bottom of the column. This inert gas may be nitrogen, air or part or all of the uncondensed gases at the top of the absorption column. The quantity introduced generally represents between 15% and 30% of the total gas input issuing from the reaction step. The drawback of this introduction of external gas is that, being sent to the absorption column to recover the acrylic acid that it contains, is added to the reaction gas which makes up its main feed, and consequently, the maximum absorption capacity of the column is thereby limited. Finally, and for equivalent at production capacity, the size of the absorption and stripping columns increases with the flow rate of external gas introduced.
A further drawback of the method of stripping by an external gas is to dilute the uncondensed gases at the top of the column and consequently cause a dilution of the part of these gases sent to the reaction step in order to recover the noble compounds that they contain (propylene, acrolein, traces of acrylic acid). Thus, for the same quantity of noble compounds recycled in the absence of external stripping gas, the flow of gases entering the reactor is increased by the introduction of this external gas. This results either in an increase in the flow rate of gas fed to the reactors, at constant reactant flow rate, causing accelerated aging of the catalysts due to the higher operating temperature to obtain the same yield performance, or, at constant flow rate of gas entering the reactors, reduced productivity by decreasing the flow rate of the reactants.
European patent EP 706 986, which describes a method of absorption by nonaqueous solvent without a desorption column, is incapable of obtaining the efficient removal of the light compounds, particularly acetic acid, claimed in the methods described with a desorption column.
Finally, the patents described in the prior art mention a water condensation section at the top of the absorption column. The condensed aqueous stream is particularly rich in polar compounds having volatilities lower than or close to that of water, particularly organic acids such as formic acid, acetic acid and acrylic acid. Due to the pollutant nature of this stream, it cannot be removed without subsequent removal treatment, generally by incineration of the organic compounds. The cost of this incineration treatment is increased by the fact that the stream to be treated is a liquid stream, consisting mainly of water.
European Patent Application EP-A2-1 125 912 teaches a method for purifying acrylic acid comprising a step of absorption by a heavy hydrophobic solvent, followed by a distillation step in a column under reduced pressure for topping, followed by distillation to obtain the acrylic acid without the solvent.
According to this method, the mass flow rate of heavy solvent is 0.2-4.5 times the mass flow rate of acrylic acid, and the stream sent to the column under reduced pressure contains acrylic acid at the rate of 18 to 75% by weight.